Effects of temperature dependent chemical potential on the Seebeck coefficient of TiS 2 thermoelectric material
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Effects of temperature dependent chemical potential on the Seebeck coefficient of TiS2 thermoelectric material Tsunehiro Takeuchi1,2 , Akio Yamamoto2, Koto Ogawa2 1 EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan 2 Department of Crystalline Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ABSTRACT The relation between chemical potential and Seebeck coefficient was investigated by using high-resolution angle resolved photoemission spectroscopy. The temperature dependence of chemical potential was experimentally determined for the n-type TiS2 thermoelectric material and compared with the measured Seebeck coefficient. We found that the temperature dependence of chemical potential of TiS2 is significantly large, and its effect on Seebeck coefficient is not negligible. This fact strongly indicates that the temperature dependence of chemical potential has to be properly understood to construct the guiding principle for developing new, practical thermoelectric materials. INTRODUCTION Thermoelectric materials have attracted considerable interest because the wasted heat emitted from factories, homes, automobiles, wastes treatment facilities, etc. can be converted into the useful electric power in thermoelectric generators. The serious problems about the running out of fossil fuels and global warming would be significantly relaxed by use of the thermoelectric generators for a variety of heat sources. However, the rather poor efficiency of energy conversion and the high-costs have prevented us from widely utilizing the thermoelectric generators. The efficiency of energy conversion of thermoelectric generator η is a function of the dimensionless figure of merit, ZT = S2σT / κ, where S, σ, and κ indicate Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. Since the efficiency η monotonically increases with the increasing magnitude of ZT, it is of great importance to develop materials possessing a large magnitude of ZT. Seebeck coefficient S is the most important one among the three thermoelectric properties to increase the magnitude of ZT, because ZT is directly proportional to the square of S while it is a first-order function of the other factors: σ and κ−1. One of the main factors to increase the magnitude of S is the temperature dependence of chemical potential, but its effects on Seebeck coefficient have not been quantitatively investigated yet. This rather poor circumstance is clearly understood by knowing the facts that the temperature dependence of Seebeck coefficient is quantitatively accounted for without considering the temperature dependence of chemical potential for many materials [1-8], while the extremely large Seebeck coefficient observable in some semiconductors [9,10] must be attributed to the temperature dependence of chemical potential. It is also very interesting to note that the large Seebeck coefficient of some highly correlated materials was often discussed in terms of the entropy of localized e
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